During metazoan development, large numbers of unwanted cells are eliminated by apoptosis, as are abnormal or dangerous cells in the adult. Correctly regulated apoptosis is essential for normal development and homeostasis of the adult. Our goal is to understand how a particular cell is selected to die at a particular time in development. Because of the extensive information available about the genes that regulate other developmental decisions in Drosophila, the study of developmental cell death in flies has the potential to provide important insight into how developmental regulators interact with the apoptotic machinery. Under aim 1, we will determine how a specific population of cells, the abdominal neuroblasts, or neural stem cells, are signaled to die. We have shown that the death of these cells requires genetic elements present in the genomic region around the Reaper gene. The Reaper gene is one of a cluster of cell death regulatory genes including grim, sickle and hid. We are dissecting this region in order to understand how the expression of cell death genes is coordinately regulated. Many of the molecular effectors of apoptosis have been identified and studied in great detail. These proteins are highly conserved from nematodes and flies through mammals. Surprisingly, the mechanistic details of how these proteins interact appear more divergent. Under aim 2, we propose to examine the role of the mitochondria in Drosophila apoptosis. We have found that expression of Reaper and Hid result in mitochondrial permeabilization. We propose strategies to identify proteins important for this mitochondrial permeabilization, and to understand its role in apoptosis in vivo. Taken together, these studies will provide important insight into the regulation and execution of apoptosis in the developing animal.